Molecular Gas and Star Molecular Gas and Star Formation in Dwarf GalaxiesFormation in Dwarf Galaxies

Alberto Alberto BolattoBolatto

Research AstronomerResearch Astronomer UC BerkeleyUC Berkeley

• Adam Leroy*Adam Leroy*• Josh Simon*Josh Simon*• Leo BlitzLeo Blitz

* Hard working grad students

Why should you care?Why should you care?“…“…Extreme properties are often sought for in Astronomy Extreme properties are often sought for in Astronomy

as one way to sharpen our understanding of as one way to sharpen our understanding of fundamental concepts…”fundamental concepts…”

Dwarf galaxies:Dwarf galaxies: are the first structures to form in are the first structures to form in

bottom-up bottom-up ΛΛCDM cosmologiesCDM cosmologies have low heavy element abundances, have low heavy element abundances,

just like primordial systemsjust like primordial systems are the simplest systemsare the simplest systems

Local dwarfs are windows onto the Local dwarfs are windows onto the high-z Universehigh-z Universe

A single-dish/interferometric surveyA single-dish/interferometric survey

MIDGetMIDGet A CO survey of A CO survey of IRAS-detectedIRAS-detected, ,

compact, nearby, northern compact, nearby, northern dwarf galaxies out to dwarf galaxies out to VVLSRLSR=1000 km s=1000 km s-1-1, with , with rotational velocities under rotational velocities under ~100 km s~100 km s-1-1

Observed 121 central Observed 121 central pointings with the Kitt Peak pointings with the Kitt Peak 12m12m

Follow up of 30+ galaxies Follow up of 30+ galaxies mapped using BIMAmapped using BIMA

Fabian Walter’s OVRO sampleFabian Walter’s OVRO sample

UASO 12m

BIMA

Two questions:Two questions:

What global properties distinguish What global properties distinguish galaxies with and without CO? galaxies with and without CO? Some of the best molecular gas Some of the best molecular gas

predictors are surprising: Lpredictors are surprising: LKK, M, Mdyndyn/L/LKK, B-K , B-K (B-V) (B-V)

Are there any differences between Are there any differences between large and dwarf galaxies in their large and dwarf galaxies in their molecular gas/star formation molecular gas/star formation properties?properties? Remarkably very few, even where some Remarkably very few, even where some

where expectedwhere expected

Distributions of detections/nondetectionsDistributions of detections/nondetections

Best Best predictors predictors of CO: of CO: LLKK, , LLBB, Hubble , Hubble Type,…Type,…

1/5 Z1/5 Z

Distributions of detections/nondetectionsDistributions of detections/nondetections

Best Best predictors predictors of CO: of CO: LLKK, , LLBB, Hubble , Hubble Type, FIR Type, FIR luminosityluminosity, , B-K B-K colorcolor, , K-K-band band mass to mass to light ratiolight ratio

One of the best predictors of CO in the One of the best predictors of CO in the survey…survey…

M/L ~ 3 (B-band) and ~ 2 (K-band)M/L ~ 3 (B-band) and ~ 2 (K-band) But the correlation is much tighter at the But the correlation is much tighter at the

low end in B light… CO nondetections are low end in B light… CO nondetections are systematically fainter in K-band!systematically fainter in K-band!

What is the driving relationship?What is the driving relationship? LLFIRFIR, L, LKK, L, LBB, B-K, , B-K,

Hubble Type, Hubble Type, Z, are all Z, are all correlatedcorrelated

Can we Can we identify a identify a driving driving parameter?parameter?

Normalizing Normalizing by Lby LKK removes removes trends and trends and minimizes minimizes dispersiondispersion

MM MM

MM MM

What is the driving relationship?What is the driving relationship? LLFIRFIR, L, LKK, L, LBB, B-K, , B-K,

Hubble Type, Z, Hubble Type, Z, are all are all correlatedcorrelated

Can we identify Can we identify a driving a driving parameter?parameter?

Normalizing by Normalizing by LLKK removes removes trends and trends and minimizes minimizes dispersiondispersion

MMmolmol/L/LKK is the is the tightest tightest correlation. correlation. Across all Across all galaxy sizes galaxy sizes MMmolmol/L/LKK~0.075~0.075

What does it mean?What does it mean? Facts:Facts:

Tightest MTightest Mmolmol correlation is with L correlation is with LKK, a proxy for , a proxy for MM** and and ΣΣ**

Correlations with MCorrelations with Mgasgas (H (HII) or M) or Mdyndyn are are considerably weakerconsiderably weaker

Taken together, suggest that what matters in the HTaken together, suggest that what matters in the H IIHH22 conversion is the amount of matter in the disk (conversion is the amount of matter in the disk (ΣΣ**)), not , not

just the amount of “stuff”just the amount of “stuff” Correlations with B-K could arise from Correlations with B-K could arise from

enhanced photodissociation/less dust in bluer enhanced photodissociation/less dust in bluer systems…systems…

……but systems with no CO tend to be but systems with no CO tend to be underluminous (for their mass) in K-band, not underluminous (for their mass) in K-band, not overluminous in B-bandoverluminous in B-band

Suggests that photodissociation plays only a secondary Suggests that photodissociation plays only a secondary role in setting the global amount of Hrole in setting the global amount of H22

This is indirect evidence in support of the This is indirect evidence in support of the local density (pressure) controlling Hlocal density (pressure) controlling HIIHH22

Are large and dwarf Are large and dwarf galaxies different in galaxies different in

their molecular their molecular gas/star formation gas/star formation

properties?properties?

The SFR vs. HThe SFR vs. H22 relationship… relationship…

1.4 GHz flux traces star formation 1.4 GHz flux traces star formation (e.g., Condon (e.g., Condon et al. 2002, Murgia et al. 2002; SFet al. 2002, Murgia et al. 2002; SFSNSNCRCRsynchrotron?)synchrotron?)

The SFR vs. HThe SFR vs. H22 relationship… relationship… 1.4 GHz flux traces star formation 1.4 GHz flux traces star formation (e.g., Condon (e.g., Condon

et al. 2002, Murgia et al. 2002; SFet al. 2002, Murgia et al. 2002; SFSNSNCRCRsynchrotron?)synchrotron?)

MIDGet and large galaxies fall on the same MIDGet and large galaxies fall on the same SFRSFR--H2H2 correlation correlation

ΣΣSFRSFR=10=10-3.4±0.1-3.4±0.1ΣΣH2H21.3±0.11.3±0.1

ΣΣSFRSFR=10=10-3.4±0.2-3.4±0.2ΣΣH2H21.4±0.21.4±0.2

The SFR vs. HThe SFR vs. H22 relationship… is independent of relationship… is independent of Z!Z!

1.4 GHz flux traces star formation 1.4 GHz flux traces star formation (e.g., Condon (e.g., Condon et al. 2002, Murgia et al. 2002; SFet al. 2002, Murgia et al. 2002; SFSNSNCRCRsynchrotron?)synchrotron?)

MIDGet and large galaxies fall on the same MIDGet and large galaxies fall on the same SFRSFR--H2H2 correlation correlation using the Galactic Xcousing the Galactic Xco!!

Attempts to correct CO-HAttempts to correct CO-H22 for metallicity for metallicity failfail

There is no segregation by inferred There is no segregation by inferred metallicitymetallicity (using Richer & McCall 1995)(using Richer & McCall 1995)

Attempts to correct CO-HAttempts to correct CO-H22 for metallicity for metallicity failfail

There is no segregation by inferred There is no segregation by inferred metallicitymetallicity (using Richer & McCall 1995)(using Richer & McCall 1995)

Corrections destroy the agreement!Corrections destroy the agreement!

Ways out of a constant Xco…Ways out of a constant Xco…

Size-Size-dependent dependent corrections to corrections to RC-SFR (e.g. RC-SFR (e.g. Bell 2003)?Bell 2003)?

Even then Even then large changes large changes in Xco are out in Xco are out of the of the questionquestion

A different A different SFR-HSFR-H22 regime regime for dwarf for dwarf galaxies?galaxies?

The sweet spot for star formation efficiency…The sweet spot for star formation efficiency… A maximum A maximum

star star formation formation efficiency at efficiency at 10101010 M M??

To a first To a first approximatioapproximation galaxy-size n galaxy-size / metallicity / metallicity corrections to corrections to LLFIRFIR and Xco and Xco cancelcancel

A large A large Xco(Z) Xco(Z) makes the makes the maximum maximum more more pronounced pronounced

SummarySummary

MMmolmol correlates very well with L correlates very well with LKK, not , not with Mwith MHIHI or M or Mdyndyn

Indirect support for a local Indirect support for a local density/pressure controlled Hdensity/pressure controlled HIIHH22 transitiontransition

Same SFR-HSame SFR-H22 relationship for dwarfs relationship for dwarfs and large galaxies, suggesting and large galaxies, suggesting constant CO-Hconstant CO-H22 for star forming gas for star forming gas despite changing metallicitydespite changing metallicity

A minimum HA minimum H22 depletion time / depletion time / maximum SF efficiency at 10maximum SF efficiency at 101010 M M??

CARMA is moving forwardCARMA is moving forward